“Tropical Storm Humberto, the eighth named storm of the 2013 Atlantic hurricane season, is here. Humberto has formed unusually far to the east, between the coast of Africa and the Cape Verde Islands,” announced Dr. Jeff Masters of Weather Underground. Tropical Storm Warnings are flying in the southern Cape Verde Islands, and Humberto’s rain bands have already arrived in capital city of Praia, where 1.46″ of rain has fallen, with sustained winds as high as 26 mph. Humberto’s west to west-northwest motion at 12 mph will keep the storm just south and west of the islands through Tuesday, but this path will be close enough to bring potentially dangerous rainfall amounts of 3 – 6 inches to the southern islands. Satellite loops show that Humberto is well-organized with plenty of spin and a growing amount of heavy thunderstorms. The models are bullish of developing Humberto into a hurricane just west of the Cape Verde Islands by Wednesday. If Humberto reaches hurricane strength before 8 am EDT on Wednesday, 2013 will avoid setting the record for the latest formation date of the Atlantic’s first hurricane, dating back the beginning of the aircraft reconnaissance era in 1944. Humberto is expected to take a sharp northwards turn later this week, which will carry the storm into a region of ocean where no land areas would likely be at risk from a strike, with the possible exception of the Azores Islands.”

For more on this story go to: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2517

EDITOR: According to the models TS Humberto poses no threat to the Cayman Islands

Related story:

The lack of Atlantic Hurricanes: the saga of low relative humidity continues

Posted by: Lee Grenci at Weather Underground

As a follow-up to my blog about the role low relative humidity in the middle troposphere (roughly 700 mb to 500 mb) plays in limiting the development of tropical cyclones (quick summary), I’ll expand my theme further to postulate that low, mid-level RH is also the primary reason for the lack of activity so far during the “heart” of hurricane season (August 15-October 15).

The average 700-mb relative humidity for the period, August 15 to September 3, 2013. Larger image. Courtesy of the Earth System Research Laboratory.

For proof, I offer the analysis of the average 700-mb relative humidity from August 15 to September 3 (above; larger image). The analysis indicates the high mean RH associated with tropical waves coming off the coast of Africa (upward motion and the associated moist-adiabatic cooling decrease relative humidity). Farther east, note the “wall” of mean low relative humidity over the Atlantic’s Main Development Region (the Atlantic’s MDR lies roughly between 10 and 20 degrees north latitude). In my opinion, there seems to be a connection between the low relative humidities in the Southern Hemisphere and this “wall.”

Specifically, note the low relative humidity (in the mean) over eastern Brazil. That’s associated with a regional spell of dry weather. To see what I mean, check out the average surface precipitation rate from August 15 to September 3. Without reservation, there’s been some dry weather over eastern Brazil, where precipitation rates have less than 2 millimeters per day during the period, August 15 – September 3 (see analysis attached).

The average surface precipitation rate for the period, August 15 to September 3, 2013. Larger image. Courtesy of the Earth System Research Laboratory.

The 15-day totals from the Climate Prediction Center (August 21 – September 4) tell a similar story. Granted, eastern Brazil tends to have sparse rainfall from June to September anyway, but I’m thinking that some of the mid-level air with low relative humidity was transported across the equator toward the “wall” by southerly winds. For confirmation, I plotted the meridional (north-south) components of the mean wind for the period August 15 – September 3 (below; speeds are expressed in meters per second; larger image). Note the positive values over eastern Brazil, indicating a southerly component to the wind.

The average meridional (north-south) components of 700-mb winds for the period, August 15 to September 3, 2013. Positive values indicate winds with a southerly component. Speeds are expressed in meters per second. Larger image. Courtesy of the Earth System Research Laboratory.

Of course, the Saharan Air Layer has also contributed to the pattern of low relative humidity in the middle levels. Check out this SAL loop (provided by CIMSS) clearly shows the westward reach of the Saharan Air Layer during the period from August 23 to September 5 (images are every six hours).

The average 700-mb vertical velocities for the period, August 15 to September 3, 2013. Positive values indicate downward motion. Negative values represent upward motion. Units are Pascals per second. Larger image. Courtesy of the Earth System Research Laboratory.

I also considered large-scale subsidence as a source of low relative humidity in the middle troposphere over the MDR (above; larger image), but, as you can see, there was a “wall” of basically neutral vertical velocities (expressed in Pascals per second) over the Main Development Region. There was a tongue of weak downward motion (in the mean) that extended northwestward across the equator at longitudes 35-40 degrees West (note that this tongue connects to the “bulls-eye” of mean downward motion centered along the equator near 20 degrees longitude West). Nonetheless, I wager that subsidence was probably not a primary source for low 700-mb relative humidity over the MDR during this period.

The Relative Humidity Kid,

Lee

For more and to find Lee Grenci’s previous post on this subject go to:

http://www.wunderground.com/blog/24hourprof/comment.html?entrynum=66

and

http://www.wunderground.com/blog/24hourprof/comment.html?entrynum=65

and

http://www.wunderground.com/blog/24hourprof/comment.html?entrynum=64